1. Field of the Invention
This invention relates generally to the making of copper and copper alloy foil and more particularly, to the making of ultra-thin foil using a copper or copper alloy carrier.
2. Background Information
Copper foil has been widely used for many years in the manufacture of printed circuits. The raw material for printed circuits usually consists of a laminate of a copper foil and a resinous substrate. In recent years, it has been found desirable to make such laminates for printed circuits using copper foil that is much thinner than formerly used. Ultra-thin foils of thicknesses of less than 18 microns (0.5 ounces per square foot) and on the order of 5-9 microns are now being used. One reason for the use of thinner foil is the demand for printed circuits with good dimensional precision, especially in circuitry with narrow width lines and short distances between the conductors. With the use of thinner copper foils, decrease of the so called "undercut" is obtained. By undercut is meant the elimination of copper under the etch resist caused by the etching solution which while dissolving the unprotected portions of the copper layer also eats into the copper covered by the etch resist.
Further, there is also a desire to reduce the amount of copper removed by the etching process which forms the circuit conductor pattern so as to reduce the cost of etchant as well as reducing copper recovery costs, or alternatively, a reduction of copper loss if no such recovery is effected.
When copper foil is made by itself in thicknesses of less than 0.5 ounce per square foot or less, it becomes very difficult to handle. During processing it often wrinkles and tears unless extreme care is exercised. Thus, according to current practice, the desired ultra-thin copper foil layer is conventionally formed on a temporary carrier surface of aluminum. The foil is then laminated onto an insulating support such as a glass fiber reinforced epoxy resin. Thereafter, the temporary carrier is removed, either by mechanically peeling the carrier off, or chemically by etching it off, leaving a copper foil surface on the support. The copper foil may then be etched or otherwise treated to produce the electrical circuit. Examples of making thin copper foil supported on an aluminum alloy substrate are described in U.S. Pat. Nos. 3,886,022; 3,936,548; 4,088,544; 4,113,576; 4,234,395; and 4,323,632.